Many oil well pumping systems use a lift pump mechanism to draw oil from a well to ground level. Lift pumps operate by shifting a plunger in an up and down, reciprocating motion within the barrel of the pump. A standing valve in the pump cylinder and a traveling valve in the plunger permit fluid to be drawn upwardly through the pump as the plunger is raised, and prevent downward flow of fluid as the plunger is lowered.
FIG. 1a depicts parts of such a conventional lift pump, and FIGS. 1b and 1c show the lift pump in the upstroke and downstroke configurations respectively. Referring to FIGS. 1a, b and c, it will be seen that, on the upstroke, the standing valve is open and the traveling valve closes. The upstroke valve configuration is caused by the low pressure situation generated in the barrel chamber as the rod moves up. The low pressure sucks fluid into the barrel chamber, through the standing valve. On the downstroke, the standing valve can close and the traveling valve is open. The downstroke valve configuration is caused by the high pressure situation generated in the barrel chamber as the rod moves down. The plunger moves freely through the fluid, in the barrel chamber, on the downstroke.
Oil well depths can be as great as 10,000 feet. A lift pump operating in such a well must be connected to a prime mover, mounted at ground level, by a sucker rod that extends the length of the well. The prime mover, typically an electrically induction driven motor, is coupled to the sucker rod by a pumping unit that translates the rotary motion of the motor into the straight line, up and down motion required to operate the sucker rod. Convenitonal pumping units comprise a simple lever (walking beam) with a rotating crank input, a cam element (horsehead), and a flexible cable that translate the pivoting motion of the walking beam into an essentially straight line, up and down output motion. FIG. 2 depicts such a conventional pumping unit.
The conventional pumping unit described above requires a relatively large walking beam to effect an adequate output stroke length. The resultant, relatively large "footprint", or area occupied by a conventional pumping unit, is particularly disadvantageous when designing off shore, platform mounted pumping mechanisms.
In addition to the space problems presented by conventional pumping units, the flexible cable required in conventional units presents reliability problems. In particular, the required wrapping up and down of the flexible cable around the horsehead, as the walking beam is pivoted back and forth, stresses and fatigues the flexible cable. Cables must therefore be periodically inspected, and replaced before they part. Parting of the cable could allow the sucker rod to descend unrestricted into the well, with the loss of the sucker rod and permanent inoperability of the well. Moreover, the cyclical rate of a pumping unit is limited by the use of a flexible cable, since the downstroke of the cable at the horsehead can at no instant be faster than the free fall descent of the sucker rod within the well. If the downstroke were faster than the descent of the sucker rod, the flexible cable would first slacken, and then would be abruptly tightened as the walking beam slowed down in readiness for the upward stroke.
Notwithstanding the above noted limitations, replacements for the conventional pumping unit having a walking beam, horsehead, and flexible cable have not been forthcoming. It will be appreciated that the loads presented to a pumping unit by a column of fluid and sucker rod that are often a mile long are extremely high and asymmetrical. Accordingly, a pumping unit designed for use with an oil well lift pump must minimize the joint and pin forces within the pumping unit, present as constant a level of torque as possible to the prime mover, and minimize force fluctuations and loading on the sucker rod. Moreover, the pumping unit should be compact in order to both reduce inertia problems and to limit the footprint of the unit. Moreover, because oil well pumping units are designed to operate continuously for months at a time, reliability is a paramount design factor which cannot be compromised in favor of other design considerations.